Not Applicable.
The invention relates generally to the surface finishing of implantable prosthetic components. More particularly, the invention relates to surface finishing of an articulating or contacting surface of a polymeric orthopedic prosthesis.
Those in the art have generally utilized, in conjunction with prosthetic implantation surgeries, components that have varying surface roughness characteristics. For example, it is currently common in the art for components such as liners, cups, stems, heads, trays and shells to be implanted in a human body with polished or as-machined surface finishes wherein machine lines or feed lines on the component(s) are visible to the unaided eye. These surface texture or roughness characteristics tend to produce wear debris that is shed from the components during a xe2x80x9cbreaking-inxe2x80x9d or xe2x80x9cwearing-inxe2x80x9d period that occurs due to, for example, micromotion at a shell to liner interface, or due to articulation at a liner to head interface.
When quantities of wear debris are generated and accumulate in an implanted prosthesis, problems may arise that compromise the longevity and/or integrity of the implanted component(s) or the entire implanted prothesis. Among these problems is the possibility of a component or components becoming loosened which, in turn, could render the component(s) or the entire prosthesis ineffective for its intended purpose, or which could cause fracture of the affected implanted component(s) or prosthesis. Other problems which could occur are local inflammation or tissue reactions, or the resorption of the bone(s) surrounding, or in the vicinity of, the implanted prosthesis. Such bone resorption could, in turn, prematurely necessitate the total replacement of the entire prosthesis via revision surgery.
Several previous attempts have been made to limit the amount of wear debris produced as a result of the xe2x80x9cbreaking inxe2x80x9d or xe2x80x9cwearing-inxe2x80x9d of components due to, among other things, micromotion and/or articulation. These attempts have focused on attempting to produce components with higher quality finishes that have less surface roughness. For example, some in the art have performed supplemental hot pressing of the polymeric components to be used in connection with prosthetic implantation surgeries in order to improve the surface finish of those components. Such supplemental hot pressing, however, has detrimental effects on the microstructure of the pressed components, and may cause delamination of articulating surfaces of ultra-high molecular weight polyethylene implantation components. Other approaches in the art have either not been able to produce components with higher quality finishes or have produced such components only with accompanying drawbacks to the performance and/or structure of the components.
Therefore, a need exists in the art for non-specialized components that have improved finishes for use in prosthetic implantation surgeries. In particular, there is a need for finished components that have a surface that reduces or substantially eliminates the formation of wear debris when the surface is subjected to contact or motion, while not adversely affecting the microstructure of the components, and which otherwise maintains beneficial aspects of prior art implantation components. A need also exists for a method to prepare such components.
This invention provides a finished implantable polymeric prosthetic joint or bone component having a superior surface finish, or xe2x80x9csuper finishxe2x80x9d, and a method for fabricating the component. The joint or bone component can be a cup, shell, tray insert, stem, liner or head, or other component which has a face that, once implanted or during implantation, is a surface which is to be subjected to a surface-to-surface contact, or motion. The joint or bone component is prepared by subjecting the component to a turning movement against a surface cutting tool such that the component has a circularly symmetric or radially symmetric finish region with an as-machined cut surface finish substantially smoother than about 0.5 micrometers. The joint or bone component, once prepared in accordance with the present invention, has a visually smooth contact region and is matable with a similarly contoured implant component for sliding or micromovement therebetween.
In an alternate embodiment of the invention, the implantable polymeric prosthetic joint or bone component has a two-dimensional surface contour with an as-machined surface profile in a cross direction with peak to valley surface smoothness effective to achieve in vivo particle generation at a residual (e.g., asymptotically flat) level or rate.
Also provided by the present invention is a method of preparing the implantable prosthetic joint or bone component. The method comprises the steps of placing the component in a mounting, such as a chuck or collet, on a turning machine, such as a lathe, for rotation against a replaceable or resharpenable cutting tool, and rotationally cutting the face of the component with a diamond cutting tool having a predetermined shape, geometry and sharpness effective to form a cut surface contour with waviness under 0.5 micrometers. The cutting tool may be brazed into a tool tip insert such as a steel or carbide tip insert, or clamped onto a tool holder for indexable tooling.
Preferably, once mounted, the component is rotated while the cutting tool is fed across a controlled R, X and Z feed path one or more times, with each time constituting one xe2x80x9cpass.xe2x80x9d Each pass removes material from the surface of the component, and the diamond cutter tool path forms a final profile. During the turning of the component by the cutting tool, the position of the cutting tool and/or the total number of passes of the cutting tool may be controlled by computer. The method of the present invention also contemplates steps of cooling the cutting edge of the tool and/or removing swarf while the cutting of the component occurs. The method concludes when the surface attains the desired shape, with a surface finish substantially smoother than 0.5 micrometers and a variation of contour waviness less than or equal to 0.5 micrometers. The method of the present invention preferably provides a smooth surface finish with excursions or variations from a nominal flat or curved surface profile lying below a threshold which is 0.05 and 0.10 micrometers.
In an alternate embodiment of the method of preparing the implantable prosthetic joint or bone component, the cutting tool may be rotated to fly cut a surface of the component while it is held stationary or translated. In still further embodiments, the turning machine carries the component on an air-bearing, and the tool compound may operate with air or hydrostatic slides and damping to minimize vibration, or to prevent transfer of vibrational artifacts to the surface finish of the component.